Need to perform a reverse DNS lookup of a particular IP address in java

Question

I need to get the DNS address, for example "http://stackoverflow.com/questions/ask". I used the following code and able to get in the form of 192.X.X.X.

  Hashtable env = new Hashtable();
  env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
  DirContext ictx = new InitialDirContext(env);
  String dnsServers = (String) ictx.getEnvironment().get("java.naming.provider.url");

  System.out.println("DNS Servers: " + dnsServers ); 

Answer 1

InetAddress ia = InetAddress.getByAddress(new byte[] {74,125,127,106});
// or 
ia = InetAddress.getByName("74.125.127.106");
System.out.println(ia.getCanonicalHostName());

Answer 2

I have taken the code linked to by @Sam DeHaan, cleaned it up a bit and tested it.

/**
 * Do a reverse DNS lookup to find the host name associated with an IP address. Gets results more often than
 * {@link java.net.InetAddress#getCanonicalHostName()}, but also tries the Inet implementation if reverse DNS does
 * not work.
 * 
 * Based on code found at http://www.codingforums.com/showpost.php?p=892349&postcount=5
 * 
 * @param ip The IP address to look up
 * @return   The host name, if one could be found, or the IP address
 */
private static String getHostName(final String ip)
 {
   String retVal = null;
   final String[] bytes = ip.split("\\.");
   if (bytes.length == 4)
   {
      try
      {
         final java.util.Hashtable<String, String> env = new java.util.Hashtable<String, String>();
         env.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
         final javax.naming.directory.DirContext ctx = new javax.naming.directory.InitialDirContext(env);
         final String reverseDnsDomain = bytes[3] + "." + bytes[2] + "." + bytes[1] + "." + bytes[0] + ".in-addr.arpa";
         final javax.naming.directory.Attributes attrs = ctx.getAttributes(reverseDnsDomain, new String[]
         {
            "PTR",
         });
         for (final javax.naming.NamingEnumeration<? extends javax.naming.directory.Attribute> ae = attrs.getAll(); ae.hasMoreElements();)
         {
            final javax.naming.directory.Attribute attr = ae.next();
            final String attrId = attr.getID();
            for (final java.util.Enumeration<?> vals = attr.getAll(); vals.hasMoreElements();)
            {
               String value = vals.nextElement().toString();
               // System.out.println(attrId + ": " + value);

               if ("PTR".equals(attrId))
               {
                  final int len = value.length();
                  if (value.charAt(len - 1) == '.')
                  {
                     // Strip out trailing period
                     value = value.substring(0, len - 1);
                  }
                  retVal = value;
               }
            }
         }
         ctx.close();
      }
      catch (final javax.naming.NamingException e)
      {
         // No reverse DNS that we could find, try with InetAddress
         System.out.print(""); // NO-OP
      }
   }

   if (null == retVal)
   {
      try
      {
         retVal = java.net.InetAddress.getByName(ip).getCanonicalHostName();
      }
      catch (final java.net.UnknownHostException e1)
      {
         retVal = ip;
      }
   }

   return retVal;
 }